Aluminum electrolysis furnaces



Sept. 4, 1962 J. SCHMITT ALUMINUM ELECTROLYSIS FURNACES 3 Sheets$heet 1 Filed Feb. 19, 1960 INVENTCR] J/JHNNESiZH/fl/TT BY AiTORNE 5.

Sept. 4, 1962 scHMn'T 3,052,619

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United States Patent Oflfice 3,052,619 ALUMINUM ELECTROLYSIS FURNACES Johannes Schmitt, Rheinfelden, Baden, Germany, assignor to Elektrokemisk A/ S, Oslo, Norway, a corporation of Norway Filed Feb. 19, 1960, Ser. No. 9,800 Claims priority, application Switzerland Feb. 27, 1959 2 Claims. (Cl. 204-243) The specification of the US. patent application Serial No. 635,874, filed January 23, 1957, now Patent No. 2,937,980, by Kurt Toma and myself discloses a method of making a self-baking continuous anode for an aluminum electrolysis furnace, according to which the continuous anode is made by superimposing unbaked blocks of an artificial carbon mixture which has been mechanically compacted, for example by extrusion in a press or by stamping or ramming, and inserting a binding agent between the surfaces to be joined. The composition of the artificial carbon mass is such that during the baking by the heat of the furnace the carbon blocks do not flow like the artificial carbon mass which is added in piece form in the known aluminum electrolysis furnace with selfbaking anodes, so-called Soderberg furnaces. The blocks soften without substantially altering in shape.

It is known how the temperature at which an artificial carbon mass composed, for example, of pulverised coke and coal tar pitch begins to flow can be increased. This can be done in the first place by reducing the content of pitch. The size of grain of the coke also has some effect and the coarser the grain the lower is the softening point. A mass of carbon-containing material in which the content of pitch amounts to 20 to 100% can be used as binding agent for sticking the blocks together. The ordinary Stiderberg mass which has a high pitch content can also be used for joining the unbaked blocks. The binding agent is a carbon-containing material which has a lower viscosity than the artificial carbon mixture of the blocks, that is to say it begins to flow at a relatively lower temperature. A material of this nature can be introduced in a pasty or liquid state by pouring it into the joints between the blocks or it may be spread over the upper surfaces of the blocks. It readily fills up any hollow spaces in the surfaces of the unbaked blocks. It can also be used in a solid condition, for example in the form of a powder. It can then be spread as such on the horizontal surfaces to be joined in a layer of a thickness of, for example, 5 to 30 mm. before the new block is superimposed on it. The vertical joints between the blocks may be made by filling them with this powder and ramming it in. When the blocks are heated in the aluminum electrolysis furnace, the powder is also heated and melts and then fills up all the cavities. A powder of this kind which is suitable for the horizontal surfaces to be joined consists, for example, of 38% pitch coke, purest coke and 52% medium hard pitch (softening point about 80 C.), and has the following grain composition:

3,052,619 Patented Sept. 4, 1962 The dry adhesive mass for the vertical joints should preferably have a coarser grain. It may be composed, for example, of 57.6% pitch coke, 14.4% purest coke and 28.0% medium hard pitch (softening point about C.) and may have the following grain sizes:

Percent 1.68-3.36 mm 16 0.84-1.68 mm-.. 28 0.42-0.84 mm 28 0.21-0.42 mm 28 EXAMPLES OF BLOCK ANODE MASSES (1) A Block Anode Mass Consisting 0 Approximately 70%. Pitch Coke and 30% Pure Coke With Hard Pitch as Binding Agent (a) COMPOSITION OF THE MASS Granulation Percentage Kind of coke according fraction of to Tyler, mm. the total mass Pitch coke 1. 68-3. 36 8 D 0 21-1. 68 30 Do 19 Pure coke 3 D 8 Do 14 Binding Agent: Hard Pitch 18 (1;) PROPERTIES OF THE HARD PITCH Softening point according to Kramer-Sarnow C 86 Coking residue percent 60 Anthracene oil insoluble do 14 Benzene insolubl do 44 (2) Block Anode Mass Consisting of 55% Pitch Coke, 30% High Temperature Coke and 15% Waste Anode Material With Hard Pitch as Binding Agent ((1) COMPOSITION OF THE MASS (1)) PROPERTIES OF THE HARD PITCH THE SAME AS THOSE OF THE BLOCK ANODE MASS (1) Anodes according to the described invention are preferably composed of at least two blocks of car-hon mass which are arranged sidebyside. The complete carbon block is preferably enclosed by an iron frame in which it can slide upwards and downwards as a whole. The anode can, however, be without a frame.

The horizontal distance apart of the carbon blocks which are placed side by side may amount to 5 to 50 mm. and is preferably from to 40 mm.

The current supply to the anode can take place through vertical steel rods, the holes for the rods being formed by semi-circular recesses in the sides of each pair of adjacent carbon blocks facing the vertical rods. With this arrangement, however, difficulties occurred during operation and it was found to be more advantageous to supply the current to the anodes from the side by horizontal metal rods or by rods inclined to the horizontal.

The supply of current by lateral rods to self-baking anodes for an aluminum electrolysis furnace is known. In these known anodes the electrode mass is contained, for example, in an aluminum casing which itself is surrounded up to a certain height by an iron frame. Openings are provided in the frame and in the aluminum casing through which the lateral current conducting rods can be pushed obliquely downwardly into the unbaked electrode mass while it is still soft. These current conducting rods are usually made of steel and are, for example, 65 cm. long and taper from 70 to 50 mm. in diameter and are pointed at the front end and flat and coated with copper at the rear end. The flat part is connected by flexible aluminum strips and a current conducting rod (anode rod) to the bus bar. Only the two lowermost rows of these exchangeable rods enter into the baked-part of the anode and are connected to the bus bar, the rows above them are not connected. As the electrode is eaten away in the molten bath and sinks downwardly the connection of the lowermost rod to the current-connecting rod is released. The rods are then drawn out of theanode and areinserted again nearer the top. The next hitherto unconnected series of the rods is connected to the current-supplying rod. With this arrangement the anode is suspended from an iron frame and the lateral contact rods are used onlyforc urentsupply. p There are various other forms of construction of continuous self-bakir1g anodesjwith 'lateralcurrent supply for the aluminum electrolysis in a fluoride bath. It is also known to suspend thejelectr'odes on thelateral contact rods so that they carry the weight of, the electrode.

My present'invention refers to an aluminum electrolysis furnace having a continuous, self-baking anode composed of unbakedand mechanically compacted artificial blocks of carbon into which contact studs Rare inserted laterally and are disposed horizontally or at a slight inclination to the horizontal and, in accordance with my inventiomthe studs of the lower rows, or at least of the lowest row, are connected to thevcurrent supply rods both electrically and so as to support the anode whereas the studs of the rows above or of the rows which are 10 to 40 cm., preferably to cm., higher are connected only electrically to the current supply rods by means of flexible conductors which do not support the anode.

.This arrangement takes into account the factor that the baking of the artificial carbon mass is not yet completed at a certain level. If the contact studs at that level are connected'rigidlylwith the current supply rods so as to support the anode,it mayhappenthat the holes in the studs widen so that the electiical'contactbetween the carbon block and the contact studs is impaired. In the arrangement according to my inventionfichis dangerdoes not occurbecause the electrical connection of the contact studs "with the current supply rods is ensured by flexible conductors so that the contact studs do not take part in'carrying the anode. The flexible conductors may for example beam the form of'bundl'es' of aluminum or copper strips.

The electrically conducting and mechanically supporting connection of the contact studs of the lower rows or row may be effected by screwing the studs with copper plated heads into the anode in the usual manner.

The holes for the contact studs are preferably produced during the manufacture of the blocks of unbaked artificial carbon mass. it is advisable always to arrange two stud holes above one another in each carbon block although more than two or only one stud hole may be provided in each carbon block. For example, particularly large carbon blocks may be provided with four stud holes two of which are in the lower half and two in the upper half.

While the anode sinks as it is used up, freshly mounted blocks are slowly heated under the influence of the temperature in the aluminum electrolysis furnace. During this heating, the temperature exceeds that of the softening point of the mass, although the block does not melt as the temperature passes through the softening point, but substantially retains its shape. The dimensions of the holes for the studs may easily alter in certain circumstances so that the contact studs which are introduced later will no longer fit sufiiciently tightly against the walls of the holes, even if the studs are coated for example with a mixture of graphite and pitch, so that the transmission of current becomes faulty.

For this reason, filling bodies of suitable material are inserted into the stud holes of the unbaked blocks according to a further feature of the invention. Filling bodies of aluminum have been found to be very satisfactory for this purpose. These filling bodies serve to preserve the shape of the stud hole until the baking is sufficiently advanced. When this state is attained, in which neither the form of the block nor the form of the stud hole can alter any more, the filling bodies are removed and replaced by the contact studs. These contact studs usually consist of steel and have a flattened, copper plated connecting head which, according to the invention, is first connected to the current supply by means of flexible conductors and later, when the row of studs has reached the lowest layer, is connected to the current supply rods both electrically and mechanically so to support the anode. I

FIGURES 1 to 4 of the accompanying drawings illustrate two constructional embodiments of my invention, in which the connecting parts are shown in FIGURES 2 and 4 partly in section and at right angles to the end views of FIGURES 1 and 3 respectively. Only a part ofthe anode is shown. Referring to the drawings, the anode blocks 1, 2 and 3 are shown, as well as the joints 4. Into the upper stud holes are inserted filling bodies 5 of aluminum, which are flush with the lateral surface of the blocks and have bars 6 at the head ends for a tool with which the filling body can later be removed. In FIGURE 2 the filling bodies 5 and the steel studs 7, 8 and 9 are not shown in their entire length of, for example, 65 cm. They pentrate into the anode blocks to a depth of, for example, 40 cm. Contact studs 7 have already replaced the aluminum filling bodies in the lower stud hole of the block 2. These contact studs 7 are, however, not yet electrically connected, as can be seen from FIGURE 1. The contact studs 8 form the lowest row but one and are electrically connected to the current supply and suspension rods by means of flexible conductors 11 consisting of aluminum strips, but are not connected to these rods in such a way as to support the anode. The contact studs 9 of the lowest row, on the other hand, are connected rigidly and, therefore, bothelectrically and mechanically to the clnrent supply rods 12 so that they support the anode. The rods 12 are connected at the top to the bus bars. The anode is therefore carried by the contact studs 9 and the current supply rods .12. It is only when the anode has been usedup to such an extent that the contact studs 9 have to bepulled out and inserted further up in placed the aluminum filling bodies that the contact studs 8, which are now in the row which has reached the lowest position, are connected to the bottom end of the current supply rods 12 in such a way as to mechanically support the anode, whereas the contact studs 7 are connected to the flexible current conductor 11.

FIGURES 3 and 4 show a dilferent form of connection to the contact studs. The contact studs 8 are electrically connected to .the current supply rod 14 by means of flexible current conductors 113 but are not connected to these supply rods so as to support the anode, whereas the contact studs 9 carry the weight of the anode.

The lower contact studs are pulled out of the anode in the usual manner when their distance from the electrolyte is only a few centimetres.

Further layers of contact higher situated studs are electrically connected to the current supply rods as the anode is eaten away, at least one layer being connected only electrically and one or more layers below it being connected so as to carry the anode.

I have found that it is advantageous to stagger the blocks in height in such a manner that one block is half a block higher or lower than the adjacent block. This arrangement is maintained with every new layer of blocks that is superimposed.

A new layer of blocks is superimposed for example when the anode has burned down at the bottom to such an extent that only 4 or 5 layers of blocks are left.

When the blocks are pressed from an unbaked mass, two, four or more stud holes are preferably also formed at the same time, for example exactly into the centre of the lower half and the centre of the upper half. Consequently the contact studs of each row will be at the same level, since one row of studs will be composed alternately of the contact studs in the lower hole of each block and the contact studs of the upper hole of each adjacent block and conversely. As a result of this, only the horizontal joint of every second block will be exposed to the molten bath when the bottom surface of the anode has burned down as far :as the joint. The FIGURES 5 and 6 illustrate this relationship. FIGURE 6 shows how far the studs extend into the carbon blocks. It also shows how the carbon blocks are staggered horizontally.

It is of course also possible to use blocks which take up the whole width or length of the anode, in the usual anodes, which are rectangular and of considerably greater length than width in cross-section, the current supply studs may be disposed either only along the two long sides or in addition at the end faces.

The claims defining my invention are as follows.

I claim:

1. In an aluminum electrolysis furnace, an anode comprising a generally vertical row of compacted carbon blocks and baked to a greater extent at a relatively lower point than at a relatively higher point, a lower contact stud at said relatively lower point, a higher contact stud at said relatively higher point, a current supply rod, an electricallyconducting and anodesupporting connection between said lower contact stud and said current supply rod, and a flexible electrically-conducting but nonsupporting connection between said higher contact stud and said current supply rod.

2. The combination set forth in claim 1 wherein said studs are received in lateral holes in said anode, said anode is formed with a contact-stud-receiving hole at a still higher point at which the baking has not proceeded to either of said extents, and the last-mentioned hole is temporarily filled with a removable filling body diiferent from said contact studs.

References Cited in the file of this patent UNITED STATES PATENTS 1,757,695 Westly May 6, 1930 2,758,964 Liles Aug. 14, 1956 FOREIGN PATENTS 786,379 Great Britain Nov. 20, 1957 786,932 Great Britain Nov. 27, 1957 UNITED STATES PATENT OFFICE vCERTIFICATE OF CORRECTION Patent No, 3,052,619 7 September 4, 1962 c Johannes Schmitt It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 33,, for "20" read 25 column 2, second table thereof, third column, line '7 thereof, for "19" read l2 same table, third column, line 8 thereof, for "12" read l9 Signed and sealed this 19th day of February 1963,,

(SEAL) Attest:

ESTON G JOHNSON DAVID L. LADD Attesting Officer I Commissioner of Patents 

1. IN AN ALUMINUM ELECTROLYSIS FURNACE, AN ANODE COMPRISING A GENERALLY VERTICAL ROW OF COMPACTED CARBON BLOCKS AND BAKED TO A GREATER EXTENT AT A RELATIVELY LOWER POINT THAN AT A RELATIVELY HIGHER POINT, A LOWER CONTACT STUD AT SAID RELATIVELY LOWER POINT, A HIGHER CONTACT STUD AT SAID RELATIVELY HIGHER POINT, A CURRENT SUPPLY ROD, 